Pretty much the only resistors I ever found bad, all by themselves, were the 100K Ohm 1/4 Watt or 1/2 Watt resistors used to jump start switching power supplies. These probably still didn't go bad for "no reason at all", they went bad because they were high resistance low power and so had a very narrow ribbon of carbon film winding around them that could get broken open easily by mechanical or electrical stress (or a latent manufacturing defect.)
Where have you seen carbon film resistors that just failed for no reason?
BTW, you will find very few (probably 0) carbon film resistors in a 555-II.
Where have you seen carbon film resistors that just failed for no reason?
BTW, you will find very few (probably 0) carbon film resistors in a 555-II.
I've had lots of copperweld lead failures.
The copper striker coat to the steel fails and the part won't conduct.
Looks like there is nothing wrong with the part, or the solder joint.
I have had caps fail, and resistors, and in various brands, Carver, Hafler, GAS (Ampzilla), etc.
Also, any part with gold leads (tin embrittlement, leading to solder joint fractures).
The copper striker coat to the steel fails and the part won't conduct.
Looks like there is nothing wrong with the part, or the solder joint.
I have had caps fail, and resistors, and in various brands, Carver, Hafler, GAS (Ampzilla), etc.
Also, any part with gold leads (tin embrittlement, leading to solder joint fractures).
Are you implying that resistor failures happen more than rarely? I've had a few 0805 SMD resistors fail in 20+ year old Sony Betacam decks. Actual resistor failures without overloading them or capacitor acid leaks onto them are extremely rare. Of course I haven't seen it all yet as I've only been doing this since '72. 'Lytic caps are a totally different story. Last year 3 of us changed over 10,000 pieces. Ceramic caps maybe 10 units over 15 years. Resistors almost never.
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Okay, here's what I got. I measured (in circuit) resistance across BE and BC for Q4 and Q7 with B+ and B-. The results were identical for the components in the good and the bad channel.
Q4 (2912):
B-C:
B+ 1.1 Mohm
B- inf.
B-E:
B+1.1 Mohm
B- inf.
Q7 (1210):
B-C:
B+1.1 Mohm
B- 1.1 Mohm
B-E:
B+ .9 Mohm
B- .9 Mohm
Given that both channels were identical, I'm not encouraged by these results. Your thoughts?
thanks...
Done. No smoke was released from the circuit (or technician).
Amp was powered on with no load. Still reads 82vdc in bad channel, stable, no blown fuse.
R16:
good channel: 598mv
bad channel: 96mv
R14:
good channel: 620mv
bad channel: 549mv
IMO, a clue. The lack of voltage on R16 leads me to no current flowing through Q4. Since it is not blowing fuses that means the collector of Q4 is also around +80. I would verify that voltage across R15 is nearly 0. If that is the case I would remove Q4 and at this point, probably just replace it and fire it up. There's a good chance you've found it. While the transistor is out, measure the resistance of R15 and R16 but I would bet they are in spec. Being the curious sort, I'd plug the transistor into a curve tracer to see what it's REALLY doing. I would however be a bit concerned why that transistor failed running at <5mA and 0.38W. If it doesn't have a heatsink on it, I would add one as 0.4W on a TO-126 case would be quite hot.
After doing this work for many years, I ALWAYS suspect the last guys work including if I'm the last guy. If the workanship is good, you shouldn't be able to tell the repair has been done -- just like auto body work.
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I was just reading an earlier thread on the 555 which jogged my memory. I've known this for so long I just assume everyone does (possibly not the noobs). When you do a 'diode check on a transistor and see 'normal' diode action of the E-B and B-C junctions, also check for E-C shorts. The first time i ran into that (1970 ?) I was a bit surprised but ALWAYS check for E-C shorts first.
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Speaking as a noob, and someone who was all of 7 in 1970, I would be very interested to have you describe what your method is for transistor testing in circuit. Between books and the Google I have amassed quite a bit of often contradictory information.
If I was a wealthy man, I would just apprentice with a tech for 6 months, but...
cheers,
Fred
There's nothing wrong with the workmanship, it's just that those two resistors (R17-18) on the good (R) channel are not the same type/brand as all the others, so clearly they were replaced at some point. I just wonder why...
Hmmm, voltage across R15:
Good channel: .1mv
Bad channel: 350mv
I'm confused. (What else is new?)
Replace Q4 and turn it on. I bet it works. Is there a heatsink on the device? The data I found (very little) on the 2SC2912 show it to be a TO-126 which has very limited junction to ambient capabilities I.E. it runs hot - too hot IMO. It shouln't take a lot of heatsink. Something like this would help a lot.
Digi-Key - HS216-ND (Manufacturer - 577500B00000G)
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